Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? There is a growing consensus among hurricane scientists that this is indeed quite possible. Two recent studies, by Zhao et al. (2009), "Simulations of Global Hurricane Climatology, Interannual Variability, and Response to Global Warming Using a 50-km Resolution GCM", and by Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", found that global warming might increase wind shear over the Atlantic by the end of the century, resulting in a decrease in the number of Atlantic hurricanes. For example, the second study took 18 relatively coarse (>60 km grid size) models used to formulate the 2007 IPCC climate report, and "downscaled" them using a higher-resolution (18 km grid size) model called ZETAC that was able to successfully simulate the frequencies of hurricanes over the past 50 years. When the 18 km ZETAC model was driven using the climate conditions we expect in 2100, as output by the 18 IPCC models, the authors found that a reduction of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century resulted. An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

Figure 1. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.

Since the Knutson et al. study using the 18 km resolution ZETAC model was not detailed enough to look at what might happen to major Category 3 and stronger hurricanes, a new study using a higher resolution model was needed. This was done by a team of modelers led by Dr. Morris Bender of NOAA's GFDL laboratory, who published their results in Science in February. The authors used the GFDL hurricane model--the model that has been our best-performing operation hurricane track forecasting model over the past five years--to perform their study. The GFDL hurricane model runs at a resolution of 9 km, which is detailed enough to make accurate simulations of major hurricanes. The researchers did a double downscaling study, where they first took the forecast atmospheric and oceanic conditions at generated by the coarse (>60 km grid) IPCC models, used these data to initialize the finer resolution 18 km ZETAC model, then used the output from the ZETAC model to initialize the high-resolution GFDL hurricane model. The final results of this "double downscaling" study suggest that although the total number of hurricanes is expected to decrease by the end of the century, we should expect an increase of 81% in the number of Category 4 and 5 storms in the Atlantic. This trend should not be clearly detectable until about 60 years from now, given a scenario in which CO2 doubles by 2100. The authors say that their model predicts that there should already have been a 20% increase in the number of Category 4 and 5 storms since the 1940s, given the approximate 0.5°C warming of the tropical Atlantic during that period. This trend is too small to be detectable, given the high natural variability and the difficulty we've had accurately measuring the exact strength of intense hurricanes before the 1980s.The region of the Atlantic expected to see the greatest increase in Category 4 and 5 storms by the year 2100 is over the Bahama Islands (Figure 2), since wind shear is not expected to increase in this region, and sea surface temperatures and atmospheric instability are expected to increase there.

The net effect of a decrease in total number of hurricanes but an increase in the strongest hurricanes should cause an increase in U.S. hurricane damages of about 30% by the end of the century, the authors compute, assuming that hurricane damages behave as they did during the past century. Over the past century, Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, Pielke et al., 2008.)

Figure 2. Expected change in Atlantic Category 4 and 5 hurricanes per decade expected by the year 2100, according to the Science paper by Bender et al. (2010).

CommentaryThese results seem reasonable, since the models in question have been successfully been able to simulate the behavior of hurricanes over the past 50 years. However, the uncertainties are high and lot more research needs to be done before we can be confident of the results. Not all of the IPCC models predict an increase in wind shear over the tropical Atlantic by 2100, so the increase in Category 4 and 5 hurricanes could be much greater. Also, the GFDL model was observed to under-predict the strength of intense hurricanes in the current climate, so it may not be creating enough Category 4 and 5 hurricanes in the future climate of 2100. On the other hand, IPCC models such as the UKMO-HadCM3 predict a very large increase in wind shear, leading to a drastic reduction in all hurricanes in the Atlantic by 2100, including Category 4 and 5 storms. So Category 4 and 5 hurricane frequency could easily be much greater or much less than the 81% increase by 2100 found by Bender et al.

The estimates of a 30% increase in hurricane damages by 2100 may be considerably too low, since this estimate assumes that sea level rise will continue at the same pace as was observed in the 20th century. Sea level rise has accelerated since the 1990s, and it is likely that this century we will see much more than than the 7 inches of global sea level rise that was observed last century. Higher sea level rise rates will sharply increase the damages due to storm surge, which account for a large amount of the damage from intense Category 4 and 5 hurricanes.

Keep in mind that while a 30% in hurricane damage by the end of the century is significant, this will not be the main reason hurricane damages will increase this century. Hurricane damages are currently doubling every ten years, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. By 2015, the authors expect the same hurricane would do $300 billion in damage. This number would increase to $600 billion by 2025 (though I think it is likely that the recent recession may delay this damage total a few years into the future.) It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing. Adoption and enforcement of strict building standards is also a must.

The authors of the GFDL hurricane model study have put together a nice web page with links to the paper and some detailed non-technical explanations of the paper.

Flood~ You may not being over blowing it. Things have set up alot like 2005. Even though another season even anywhere near that active, we've been told we shouldn't see again in our life.. things are obviously changing & these 1 in 100, 1000 & even 10,000 year events are turning out not so 1 in.. with repeats being more the new norm. I'm somewhere between 15 & 19. I think Joe's 7 landfalls is a bit high especially with the uncertainty of ENSO right now. If it crashes hard the Caribbean should get nailed, not so much the US.

Can't say it enough if you live in an area vulnerable to tropical systems NOW is the time to get ready. Just about every parameter is coming togther to produce a well above average year so dont wait till the storm is knocking on your doorstep.

we have had a load of rain after seeing temps in high 70's for almost 2 weeks now back to normal with temps in mid 30's this am overnight down to high 20's brrrr just a 36 hr cold spell then warm up next week be back to near 70 by mid week

Quoting CycloneOz:It's a little bit funny, when you get bannedHow you get back in, we do not understandI'm not a WU admin, but if I wasI'd IP you out so hard, your head would buzz

I've tried to ignore you, but you defeat me againNow I'm ready to stab myself in the head with a penYou're back one more time and I feel myself heaveNo matter what you call yourself now, you're still JFV

And you can tell everybody, it's really not youSome may believe, but I'm not a foolI know it's you, manI know it's you, man...we sniff you out like a dogHow wonderful life is, when you're not on the blog

Quoting Cane2010:Morning, Baha, long time no see, sir; how you've been?

Quoting Cane2010:Thanks, and yes, it is. I'm just not him; whoever he is, :(. Why can't they understand that, sigh.

its you you need toos stop lieing be thats is this what you are doing i can tell its you be come the way you spell and the way you talk and the way you all ways : Quote some one evere time they make a commet about some in

If you actually read what JB says on his blogs... he is predicting 2010 season more like 2008 in terms to track but just more of it...!! IKE and Gustav comes to mind...!And if that comes true then the Gulf coast will be in trouble!!

Quoting IKE:He's calling for 7 land falling TS's and canes in the USA in 2010....Link

This blog will explode this summer.

I am concerned. Our country has enough going on without getting hit by one (or more) hurricanes. I believe if it happens there will be serious set backs for us. There will also be unforeseen repercussions that I am sure we could live without. I am making my prediction May-15. Maybe I will get it right this year.

Quoting hydrus: Hey Ike- Up here on the plateau, we had a low temp at- 64, high temp-89. Now we have a low of-37, and the high temp at 64. I like it because I dont have to run the heat or the a/c. It looks like someone is getting a hurricane this year. Where? Who knows.

Looks like a bad year ahead. Probably starting early.

I haven't read Bastardi's take on the season. I've seen his numbers and where they may form/go.

Hey Ike- Up here on the plateau, we had a low temp at- 64, high temp-89. Now we have a low of-37, and the high temp at 64. I like it because I dont have to run the heat or the a/c. It looks like someone is getting a hurricane this year. Where? Who knows.

Quoting CycloneOz:It's a little bit funny, when you get bannedHow you get back in, we do not understandI'm not a WU admin, but if I wasI'd IP you out so hard, your head would buzz

I've tried to ignore you, but you defeat me againNow I'm ready to stab myself in the head with a penYou're back one more time and I feel myself heaveNo matter what you call yourself now, you're still JFV

And you can tell everybody, it's really not youSome may believe, but I'm not a foolI know it's you, manI know it's you, man...we sniff you out like a dogHow wonderful life is, when you're not on the blog

on storm surge.. I lived directly on Indian River in Jensen Beach atop a 12 ft. bluff with what once was a pristine grass flats behind my cottage. Surge wasn't necessarily my fear, erosion of the bluff was so I left for Francis and Jeanne. If you will recall, Francis sat offshore pounding the Bahama's before she decided to come ashore in Florida. We had an onshore wind for days and this packed a lens of water against the coast, raising the water levels in Indian River well before the storm arrived. The astounding thing to me was how long it took the water level to recede after the storm. We were above normal level for two full days after the storm. I could not help but be amazed at how large the lens of high water must have been to prolong this phenomenon. Now, I suppose that out flowing fresh water from the St. Lucie could have affected this but I lived there long enough to experience some pretty serious high fresh water events coupled with the despised Okeechobee Lake discharges and never did I see river levels affected. We have a real big hole to the Atlantic just a mile and a half to the south called the St. Lucie inlet. I am a dedicated and studious observer of my physical environment and watch tides closely (I fished every day )so I am reasonably certain that what I observed was Francis related.